I find this a difficult question as I like both.
Oranges are "safer" in that they are mostly always sweet and nice.
Mandarins are much easier to eat, however they are not always nice.

I far prefer the seedless variety of mandarin and would eat more mandarins than oranges when they are in season, however this is possibly because they are so much easier to eat and I eat them at night while watching TV.

It's a tupperware orange peeler and it really works. My mother was expert at peeling an orange with no mess so I had to uphold the family honour. The "other" end is perfect for applying a thin line of wax to eyebrows.

I much prefer mandarins, as they are small and so easy to eat, especially if you are on the go. Something that's so important if you are a busy person. You can usually get lovely sweet ones if you go to the markets, and at a sweet price ! The ones at the shops are a bit hit and miss.

Finy has said what I feel on this. I would consider Mandarines one of my favourite fruit. But if I had a good orange and a good mandarine prepped ready to eat - I would have trouble choosing. Possibly would choose a mandarine as their season is shorter.
Sometimes we get a variety of Mandarine that is full of seeds. My husband and I compete to see who has the most seeds. 39 one day! I would have preferred to eat an orange.

Oranges. Never been a fan of the taste of mandarins. I may have mentioned that my mother used to set tinned mandarins in jelly. OMG how I hated, loathed and despised it. Anyway, with my trusty tupperware orange peeler I can be into an orange as quick as peeling a mandarin . . .with no mess. Brilliant tool and I use the flat end for waxing my eyebrows. LOL

Navel oranges don't have seeds and aren't quite as juicy as the ones with seeds. The varieties with seeds are better for juicing but the ones without seeds like navel oranges and valencias are easier to peel and eat.

Mandarins,& then naval oranges, but I haven't bought oranges since I used to buy by them in the orange net bags, many years ago. I don't think they're really fresh, buying them that way, as they're kept in cold storage for to long. Now I'll only buy 1 for a recipe. Perhaps all fruit is kept in cold storage for weeks or months on end!